1. Field of the Invention
The present invention relates generally to an eddy current measuring system, and in particular to an eddy current measuring system for estimating the thickness of conductive films formed on semiconductor wafer products.
2. Description of the Related Art
In the semiconductor industry, critical steps in the production of semiconductor wafers are the selective formation and removal of films on an underlying substrate. The films are made from a variety of substances, and can be conductive (for example, metal or a magnetic ferrous conductive material) or non-conductive (for example, an insulator or a magnetic ferrite insulating material).
Films are used in typical semiconductor processing by: (1) depositing a film; (2) patterning areas of the film using lithography and etching; (3) depositing material which fills the etched areas; and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP). Films may be formed on a substrate by a variety of well-known methods including physical vapor deposition (PVD) by sputtering or evaporation, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and electrochemical process (ECP). Films may be removed by any of several well-known methods including chemical-mechanical polishing (CMP), reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching, and spray etching.
The semiconductor fabrication industry continues to demand higher yields and shorter fabrication times, while insisting upon ever-increasing quality standards. A variety of inspection procedures have been employed during the various stages of the semiconductor wafer fabrication process in an attempt to meet these demands. These inspection procedures include destructive, as well as nondestructive, testing methods for analyzing wafer products.
In a destructive measuring process, a standard or electron microscope may be used to measure the thickness of a wafer""s coating after a cross-section has been obtained. When the thickness of a thin-film coating is greater than 10,000 xc3x85, for example, this type of destructive measuring method may provide accurate measurements. However, measuring accuracy usually begins to degrade as the coating thickness falls below the 10,000 xc3x85 threshold.
Other types of measuring processes utilize sensitive eddy current sensors which do not destroy or significantly alter the article measured. Although eddy current sensors provide highly accurate readings, these sensors are susceptible to error. For example, the shifting of an electronic reference point due to thermal drifting often occurs at some point during the data collection and inspection process. To compensate for thermal drifting and to ensure accurate readings, many existing eddy current sensors must be recalibrated on a periodic basis.
While there have been other attempts in addition to eddy current sensors to employ highly accurate, nondestructive measuring devices for estimating the thickness of a conductive top layer formed on a semiconductor wafer product, improvement is still needed.
A standalone eddy current monitoring system provides a thickness profile of a substrate sample by obtaining initial and terminating resistance and reactance measurements from the sample. Initial eddy current measurement values are obtained while an eddy current probe is positioned at an initial distance relative to the substrate sample, and terminating values are obtained while the eddy current probe is positioned at a modified distance relative to the sample. An intersecting line can be calculated using the initial and terminating resistance and reactance measurements. An intersecting point between a previously defined natural intercepting curve and the intersecting line may also be determined. A reactance voltage of the intersecting point may be located along a digital calibration curve to identify a closest-two of a plurality of calibration samples. The conductive top layer thickness of the substrate sample can then be determined by approximating a location, using linear or nonlinear calculations, of the reactance voltage relative to the closest-two of the plurality of calibration samples.